Access channel slot sharing

ABSTRACT

The present invention is a system and method for increasing user capacity on a slotted random access channel in a spread spectrum communications system by using a multi-part access probe. First and second parts of the access probe are modulated using a short PN code sequence, and the entire access probe is modulated using a long PN code sequence. Information to be transmitted by the access probe is modulated on the second part of the access probe, and the access probe is transmitted so that the first part of the probe falls within the boundaries of an access channel slot. In one embodiment, time slots in access channels used for access signal reception are made the length of the first part. In a further embodiment, time slots in a plurality of adjacent access channels used for access signal reception may be longer than said first part but are offset in time from each other by the length or period of the first part.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is related to a commonly-owned application,filed Jun. 16, 1998, entitled “Rapid Signal Acquisition AndSynchronization For Access Transmissions” having application Ser. No.09/098,631, which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] The present invention relates generally to multiple access,spread spectrum, communication systems and networks. More particularly,the present invention relates to increasing user access capacity in aspread spectrum communication system.

[0004] II. Related Art

[0005] A variety of multiple access communication systems and techniqueshave been developed for transferring information among a large number ofsystem users. However, spread spectrum modulation techniques, such asthose used in code division multiple access (CDMA) communication systemsprovide significant advantages over other modulation schemes, especiallywhen providing service for a large number of communication system users.Such techniques are disclosed in the teachings of U.S. Pat. No.4,901,307, which issued Feb. 13, 1990 under the title “Spread SpectrumMultiple Access Communication System Using Satellite or TerrestrialRepeaters,” and U.S. Pat. No. 5,691,974, which issued Nov. 25, 1997,under the title “Method and Apparatus for Using Full SpectrumTransmitted Power in a Spread Spectrum Communication System for TrackingIndividual Recipient Phase Time and Energy,” both of which areincorporated herein by reference.

[0006] The above-mentioned patents disclose multiple accesscommunication systems in which a large number of generally mobile orremote system users each employ at least one transceiver to communicatewith other system users or users of other connected systems, such as apublic telephone switching network. The transceivers communicate throughgateways and satellites, or terrestrial base stations (also sometimesreferred to as cell-sites or cells).

[0007] Base stations cover cells, while satellites have footprints (alsoreferred to as “spots”) on the surface of the Earth. In either system,capacity gains can be achieved by sectoring, or subdividing, thegeographical regions being covered. Cells can be divided into “sectors”by using directional antennas at the base station. Similarly, asatellite's footprint can be geographically divided into “beams,”through the use of beam-forming antenna systems. These techniques forsubdividing a coverage region can be thought of as creating isolationusing relative antenna directionality or space division multiplexing. Inaddition, provided there is available bandwidth, each of thesesubdivisions, either sectors or beams, can be assigned multiple CDMAchannels through the use of frequency division multiplexing (FDM). Insatellite systems, each CDMA channel is referred to as a “sub-beam,”because there may be several of these per “beam.”

[0008] In communication systems employing CDMA, separate links are usedto transmit communication signals to and from a gateway or base station.A forward link refers to the base station- or gateway-to-user terminalcommunication link, with communication signals originating at thegateway or base station and transmitted to a system user, or users. Areverse link refers to the user terminal-to-gateway or -base stationcommunication link, with communication signals originating at a userterminal and transmitted to the gateway or base station.

[0009] The reverse link is comprised of at least two separate channels:an access channel and a reverse traffic channel. Generally, there areseveral access and reverse link traffic channels in a communicationsystem. An access channel is used by one or more user terminals,separated in time, to initiate or respond to communications from agateway or base station. Each such communication process is referred toas an access signal transmission or as an “access probe.” The reversetraffic channels are used for the transmission of user and signalinginformation or data from user terminals to one or more gateways or basestations during a “call” or communication link setup. One structure orprotocol for access channels, messages, and calls is illustrated in moredetail in the Telecommunications Industry Association IS-95 standardentitled “Mobile Station-Base-Station Compatibility Standard ForDual-Mode Wideband Spread Spectrum Cellular System,” which isincorporated herein by reference.

[0010] In a typical spread-spectrum communication system, one or morepreselected pseudo-noise (PN) code sequences are used to modulate or“spread” user information signals over a predetermined spectral bandprior to modulation onto a carrier for transmission as communicationsignals. PN spreading, a method of spread-spectrum transmission that iswell known in the art, produces a signal for transmission that has abandwidth much greater than that of the data signal. In the basestation- or gateway-to-user terminal communication link, PN spreadingcodes or binary sequences are used to discriminate between signalstransmitted by different base stations or over different beams, as wellas between multipath signals. These codes are typically shared by allcommunication signals within a given cell or sub-beam. In somecommunication systems, the same set of PN spreading codes are used inthe reverse link for both the reverse traffic channels and the accesschannels. In other proposed communication systems, the forward link andthe reverse link use different sets of PN spreading codes.

[0011] Generally, the PN spreading is accomplished using a pair ofpseudonoise (PN) code sequences to modulate or “spread” informationsignals. Typically, one PN code sequence is used to modulate an in-phase(I) channel while the other PN code sequence is used to modulate aquadrature-phase (Q) channel in a technique commonly referred to asquadrature phase-shift keying (QPSK). The PN spreading occurs beforeinformation signals are modulated by a carrier signal and transmittedfrom the gateway or base station to the user terminal as communicationsignals on the forward link. The PN spreading codes are also referred toas short PN codes because they are relatively “short” when compared withother PN codes used by the communication system. Typically, the same setof PN spreading codes are shared by the forward and reverse link trafficchannels and another set of PN spreading codes are used for the accesschannels as discussed above.

[0012] A particular communication system may use several lengths ofshort PN codes depending on whether the forward link or the reverse linkchannels are being used. In the forward link, such as in a satellitesystem, the short PN codes typically have a length from 2¹⁰ to 2¹⁵chips. These short PN codes are used to discriminate between the varioussignal sources, such as gateways, satellites, and base stations. Inaddition, timing offsets within a given short PN code are used todiscriminate between beams of a particular satellite, or cells andsectors in terrestrial systems.

[0013] In a proposed satellite communication system, the short PN codesused in the reverse link have a length on the order of 2⁸ chips. Theseshort PN codes are used to enable a gateway or base station receiver toquickly search out user terminals that are trying to access thecommunication system without the complexity associated with the “longer”short PN codes used in the forward link. For purposes of thisdiscussion, “short PN codes” refer to these short PN code sequences (2⁸)to be used in the reverse link.

[0014] Another PN code sequence, referred to as a channelizing code, isused to discriminate between communication signals transmitted bydifferent user terminals on the reverse link within a cell or sub-beam.The PN channelizing codes are also referred to as long codes becausethey are relatively “long” when compared with other PN codes used by thecommunication system. The long PN code typically has a length on theorder of 2⁴² chips, but may be shorter or masked as desired. Typically,an access message is modulated by the long PN code prior to beingmodulated by the short PN code and subsequently transmitted as an accessprobe or signal to the gateway or base station. However, the short PNcode and the long PN code may be combined prior to modulating orspreading the access message.

[0015] When a receiver at the gateway or base station receives theaccess probe, the receiver must despread the access probe to obtain theaccess message. This is accomplished by forming hypotheses, orpredictions, as to which long PN codes and which short PN code pair wereused to modulate the access message. A correlation between a givenhypothesis and the access probe is generated to determine whichhypothesis is the best estimate for the access probe. The hypothesisthat produces the greatest correlation, generally relative to apredetermined threshold, is selected as a hypothesis of the most likelycode and timing match. Once the selected hypothesis is determined, theaccess probe is despread using the selected hypothesis to obtain theaccess message.

[0016] In a communications system having many users, it is likely thatmore than one access probe will arrive at a gateway or base stationsimultaneously, or within a preselected period of time over which thesignal is to be detected. When this happens, the access probes cancollide or mutually interfere, rendering them unrecognizable to thegateway or base station. One way to avoid such collisions is to employ acentrally-controlled access technique, where the communications systemschedules user terminal access probe transmissions. One disadvantage ofsuch a technique is that a significant amount of access channelbandwidth is consumed by such a scheduling mechanism.

[0017] Another technique used to avoid such collisions is the slottedrandom access technique, such as the “slotted ALOHA” technique. In theslotted random access technique, a regular system-wide timing structureestablishes permissible transmission or reception times. The accesschannel is usually divided into a series of fixed length frames or time“slots” or windows, each having the same fixed duration slots used forreceiving signals. The access signals are generally structured as“packets”, that consist of a preamble and a message portion, that mustarrive at the beginning of a time slot to be acquired. A user terminaltransmits at its own discretion, but is constrained to transmit onlywithin the boundaries of a single slot to have a message received. Theuse of this technique on the access channel significantly decreases thepossibility that access probes from different users will collide at agateway or base station.

[0018] Unfortunately, the slotted random access technique also resultsin a significant amount of unused time on the access channel. Because anaccess probe must be transmitted within a single slot, the slot durationmust be chosen to exceed the duration of the longest possible accessprobe. Because all slots are of the same duration, a slot will bepartially empty for all but the longest access probe. The result is asubstantial amount of wasted bandwidth on the access channel and aconsequent reduction in the user capacity of the access channel.

[0019] A failure to acquire an access probe during a particular frameperiod results in the transmitter desiring access having to re-send theaccess probe to allow the receiver to detect the probe again during asubsequent frame. Multiple access signals arriving together “collide”and are not acquired, requiring both to be resent. In either case, thetiming of subsequent access transmissions when the initial attempt failsis based on a delay time equal at a minimum to the length of the timeslots, and generally to a random number of time slots or frames.Therefore, a significant amount of time passes before an access probecan again be resent and received. The length of the delay in probeacquisition is increased by any delay in resetting acquisition circuitsin the receiver to scan the various hypothesis, and in other probesbeing acquired first, as mentioned. Ultimately, the access probe maynever, at least not within a practical time limit, be acquired if thetiming uncertainty is not resolved.

[0020] What is needed is a system and method for increasing usercapacity on a slotted random access channel in a spread spectrumcommunication system. It is preferable that the technique allow accessprobes to be received with minimum delay and efficiency.

SUMMARY OF THE INVENTION

[0021] The present invention is a system and method for increasing usercapacity on a slotted random access channel in a spread spectrumcommunications system using a multi-part access probe. The presentinvention also has the advantage that it reduces delays in achievingaccess after an initial access failure.

[0022] The invention is realized in a method and apparatus fortransmitting a plurality of access signals over at least one accesschannel, each including preamble and message portions with the preamblehaving first and second stages. The access probe preamble does notcontain message information but is comprised of null data.

[0023] The access signal is generated by modulating the first stage andsecond stages of the preamble by a first signal; modulating the secondstage of the preamble also by a second signal; and modulating themessage with said first signal and said second signal. The access signalis then transmitted in the form of the modulated first stage, secondstage, and message. The access signals thus formed can be transmittedand received over an access channel divided into time slots so that thepreamble falls within one of a plurality of preselected time slots. Theresult is that when more than one access signal is transmitted in timesuch that a second stage or message portion overlaps the first stage ofone or more other transmitted access signals, it can still be acquired.

[0024] In a preferred embodiment, the access signals can be transmittedand received over an access channel divided into signal reception timeslots that are substantially the same length as said first stage.Alternatively, the access signals can be received over a plurality ofaccess channels divided into signal reception time slots that are timeoffset from each other by a period substantially the same length as saidfirst stage.

[0025] The first part of the access probe is preferably formed by firstmodulating or spreading the access signal using a short PN sequence,which is also used to spread the second part. In a preferred embodiment,the short PN sequence is a pair of quadrature short PN sequences. Thisspreading is generally accomplished using apparatus for transmitting themulti-part access probe having first and second PN code modulators, adata modulator, and a transmitter.

[0026] The first PN code modulator spreads first and second parts of theaccess probe with the desired short PN sequence while the second PN codemodulator spreads the second part of the access probe with a long PNsequence. The data modulator modulates the second part with the accessmessage. The transmitter then transmits the access probe so that thefirst part falls within one of the access channel slots.

[0027] The apparatus for receiving the multi-part access probe includesa plurality of demodulators and a searcher receiver. The searcherreceiver acquires the first part of the access probe and transfersfurther processing of the probe, that is the second part, to one of thedemodulators. The searcher receiver can then acquire the first part ofanother access probe while the demodulator demodulates the second partof the first access probe. This process can be repeated, acquire andhand-off, for as many access probes as can be received, demodulated andcan be acquired, during any given time interval.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The present invention is described with reference to theaccompanying drawings in which like reference numbers indicate identicalor functionally similar elements, and the left-most digits of areference number identify the drawing in which the reference numberfirst appears.

[0029]FIG. 1 illustrates an exemplary wireless communication systemconstructed and operating according to one embodiment of the presentinvention.

[0030]FIG. 2 illustrates an exemplary implementation of communicationlinks used between a gateway and a user terminal in the communicationsystem of FIG. 1.

[0031]FIG. 3 illustrates the structure of an access channel in moredetail.

[0032]FIG. 4 is a timing diagram depicting a typical timing structurefor access probes in a conventional slotted random access channel.

[0033]FIG. 5 is a timing diagram for access probes in a slotted randomaccess channel according to a preferred embodiment of the presentinvention.

[0034]FIG. 6 illustrates a protocol for generating an access probeaccording to one embodiment of the present invention.

[0035]FIG. 7 is a block diagram for an exemplary access channeltransmitter used for transmitting an access probe according to oneembodiment of the present invention.

[0036]FIG. 8 is a flowchart of the operation of an access channeltransmitter according to one embodiment of the present invention.

[0037]FIG. 9 is a block diagram for an exemplary access channel receiverfor receiving an access probe according to one embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] The present invention is a system and method for increasing usercapacity on a slotted random access channel in a spread spectrumcommunications system by using a multi-part access probe. The presentinvention also decreases delay in resending unsuccessful access probesor signals. In one embodiment of the present invention, the access probeis transmitted from a user terminal to a gateway or base station.

[0039] Although the invention is described in detail in terms ofspecific embodiments, various modifications can be made withoutdeparting from the scope of the invention. For example, the invention isequally suited for transmissions other than access channel transmissionsthat are spread with multiple PN code sequences. Further, thecommunications channel of the present invention is not limited to theair link described, but can be employed over wire, fiber optic cable,and the like.

[0040] In a typical CDMA communication system, a base station within apredefined geographical region, or cell, uses several spread spectrummodems or transmitter and receiver modules to process communicationsignals for system users within the service area. Each receiver modulegenerally employs a digital spread spectrum data receiver and at leastone searcher receiver as well as associated demodulators and the like.During typical operations, a particular transmitter module and aparticular receiver module, or a single modem, in the base station areassigned to a user terminal to accommodate transfer of communicationsignals between the base station and the user terminal. In some cases,multiple receiver modules or modems may be used to accommodate diversitysignal processing.

[0041] For communication systems employing satellites, the transmitterand receiver modules are generally placed in base stations referred toas gateways that communicate with system users by transferringcommunication signals through the satellites. In addition, there may beother associated control centers that communicate with the satellites orthe gateways to maintain system-wide traffic control and signalsynchronization.

[0042] I. System Overview

[0043] An example of a wireless communication system constructed andoperating according to the present invention is illustrated in FIG. 1. Acommunication system 100 utilizes spread spectrum modulation techniquesin communicating with user terminals (shown as user terminals 126 and128). In terrestrial systems, communication system 100 communicates withmobile stations or user terminals 126 and 128 using base stations (shownas base stations 114 and 116). Cellular telephone type systems in largemetropolitan areas may have hundreds of base stations 114 and 116serving thousands of user terminals 126 and 128.

[0044] In satellite-based systems, communication system 100 employssatellite repeaters (shown as satellites 118 and 120) and systemgateways (shown as gateways 122 and 124) to communicate with userterminals 126 and 128. Gateways 122 and 124 send communication signalsto user terminals 126 and 128 through satellites 118 and 120.Satellite-based systems generally employ fewer satellite repeaters toservice more users over a larger geographical region than comparableterrestrial systems.

[0045] Mobile stations or user terminals 126 and 128 each have orcomprise a wireless communication device such as, but not limited to, acellular telephone, a data transceiver or a transfer device (e.g.,computers, personal data assistants, facsimile). Typically, such unitsare either hand-held or vehicle mounted as desired. While these userterminals are discussed as being mobile, it is also understood that theteachings of the invention are applicable to fixed units or other typesof terminals where remote wireless service is desired. This latter typeof service is particularly suited to using satellite repeaters toestablish communication links in many remote areas of the world. Userterminals are also sometimes referred to as subscriber units, mobileunits, mobile stations, or simply “users,” “mobiles,” or “subscribers”in some communication systems, depending on preference.

[0046] Exemplary user terminals are disclosed in U.S. Pat. No. 5,691,974referenced above, and U.S. Pat. No. 5,835,847 which issued Nov. 10, 1998and is entitled “Pilot Signal Strength Control For A Low Earth OrbitingSatellite Communications System,” and application Ser. No. 08/723,725entitled “Unambiguous Position Determination Using Two Low-Earth OrbitSatellites,” which are incorporated herein by reference.

[0047] It is contemplated for this example that satellites 118 and 120provide multiple beams within ‘spots’ that are directed to coverseparate generally non-overlapping geographic regions. Generally,multiple beams at different frequencies, also referred to as CDMAchannels, ‘sub-beams’ or FDM signals, frequency slots, or channels, canbe directed to overlap the same region. However, it is readilyunderstood that the beam coverage or service areas for differentsatellites, or antenna patterns for terrestrial cell-sites, may overlapcompletely or partially in a given region depending on the communicationsystem design and the type of service being offered, and space diversitymay also be achieved between any of these communication regions ordevices. For example, each may provide service to different sets ofusers with different features at different frequencies, or a givenmobile unit may use multiple frequencies and/or multiple serviceproviders, each with overlapping geophysical coverage.

[0048] As illustrated in FIG. 1, communication system 100 generally usesa system controller and switch network 112, also referred to as a mobiletelephone switching office (MTSO) in terrestrial systems, and (Ground)Command and Control centers (GOCC) for satellite systems, which alsocommunicate with the satellites. Such controllers typically includeinterface and processing circuitry for providing system-wide control forbase stations 114 and 116 or gateways 122 and 124 over certainoperations including PN code generation, assignments, and timing.Controller 112 also controls routing of communication links or telephonecalls among a public switched telephone network (PSTN), and basestations 114 and 116 or gateways 122 and 124, and user terminal 126 and128. However, a PSTN interface generally forms part of each gateway fordirect connection to such communication networks or links.

[0049] The communication links that couple controller 112 to varioussystem base stations 114 and 116 or gateways 122 and 124 can beestablished using known techniques such as, but not limited to,dedicated telephone lines, optical fiber links, and microwave ordedicated satellite communications links.

[0050] While only two satellites are illustrated in FIG. 1, thecommunication system generally employs multiple satellites 118 and 120traversing different orbital planes. A variety of multi-satellitecommunication systems have been proposed including those using aconstellation of Low Earth Orbit (LEO) satellites for servicing a largenumber of user terminals. However, those skilled in the art will readilyunderstand how the teachings of the present invention are applicable toa variety of both terrestrial and satellite system configurations.

[0051] In FIG. 1, some of the possible signal paths for communicationlinks between base stations 114 and 116 and user terminals 126 and 128are illustrated as lines 130, 132, 134, and 136. The arrowheads on theselines illustrate exemplary signal directions for the link, as beingeither a forward or a reverse link, and serve as illustration only forpurposes of clarity and not as any restriction on the actual signalpattern.

[0052] In a similar manner, signal paths for communication links amonggateways 122 and 124, satellite repeaters 118 and 120, and userterminals 126 and 128 are illustrated as lines 146, 148, 150, and 152for gateway-to-satellite links and as lines 140, 142, and 144 forsatellite-to-user links. In some configurations, it may also be possibleand desirable to establish direct satellite-to-satellite linksexemplified by line 154.

[0053] As will be apparent to one skilled in the art, the presentinvention is suited for either terrestrial-based systems orsatellite-based systems. Thus, gateways 122 and 124 and base stations114 and 116 will henceforth be collectively referred to as gateway 122for clarity. The terms base station and gateway are sometimes usedinterchangeably in the art, with gateways being perceived as specializedbase stations that direct communications through satellites. Likewise,satellites 118 and 120 will be collectively referred to as satellite118, and user terminals 126 and 128 will be collectively referred to asuser terminal 126.

[0054] II. Communication Links

[0055]FIG. 2 illustrates an example implementation of communicationlinks used between a gateway 122 and a user terminal 126 incommunication system 100. Two links are employed in communication system100 to facilitate the transfer of communication signals between gateway122 and user terminal 126. These links are referred to as a forward link210 and a reverse link 220. Forward link 210 handles transmissionsignals 215 that are transmitted from gateway 122 to user terminal 126.Reverse link 220 handles transmission signals 225 that are transmittedfrom user terminal 126 to gateway 122.

[0056] Forward link 210 includes a forward link transmitter 212 and aforward link receiver 218. In one embodiment, forward link transmitter212 is implemented in gateway 122 according to well-known CDMAcommunication techniques as disclosed in the above-referenced patents.In one embodiment, forward link receiver 218 is implemented in userterminal 126 according to well-known CDMA communication techniques asdisclosed in the above referenced patents.

[0057] Reverse link 220 includes a reverse link transmitter 222 and areverse link receiver 228. In one embodiment, reverse link transmitter222 is implemented in user terminal 126. In one embodiment, reverse linkreceiver 228 is implemented in gateway 126.

[0058] As discussed above, reverse link 220 uses at least two channels,including one or more access channels and one or more reverse trafficchannels. These channels may be implemented by separate receivers or thesame receiver operating in distinct modes. As discussed above, an accesschannel is used by user terminals 126 to initiate, or respond to,communications with gateway 122. A separate access channel is requiredat any given time for each active user. In particular, access channelsare time-shared by several user terminals 126 with transmissions fromeach active user being separated in time from one another. The structureof access channels and signals is discussed in further detail below.

[0059] Systems may employ more than one access channel depending uponknown factors such as a desired level of gateway complexity and accesstiming. In a preferred embodiment, 1 to 8 access channels are employedper frequency. In preferred embodiments, different sets of PN spreadingcodes are used between the reverse traffic channels and the accesschannels. In addition, the access channels can employ very short PNcodes, chosen from a unique set of codes (or code generators), assignedonly for the use of access channels throughout communication system 100.This latter technique provides a very efficient mechanism for quicklyacquiring access signals at gateways in the presence of signal delay andDoppler and other known effects.

[0060] III. Access Channel

[0061]FIG. 3 illustrates an access channel 300 in further detail. Accesschannel 300 includes an access channel transmitter 310, an accesschannel receiver 320, and an access signal or probe 330. Access channeltransmitter 310 can be included in reverse link transmitter 222described above. Access channel receiver 320 can be included in reverselink receiver 228 described above.

[0062] Access channel 300 is used for short signaling message exchangesincluding call origination, responses to pages, and registrationsoriginated from user terminal 126 and destined for gateway 122. In orderfor user terminal 126 to initiate or respond to communications withgateway 122 over access channel 300, a signal referred to as accessprobe 330 is sent.

[0063] An access channel is also generally associated with one or moreparticular paging channels used in the communication system. This makesresponses to paging messages more efficient in terms of the systemknowing where to look for user terminal access transmissions in responseto pages. The association or assignment may be known based on a fixedsystem design, or indicated to user terminals within the structure ofpaging messages.

[0064] IV. Timing Uncertainty in Access Probe

[0065] An uncertainty in the timing of access probe 330 arises due tothe changing distance or propagation path length between user terminal126 and satellite 118 as a result of the orbit of satellite 118 aroundthe Earth. This timing uncertainty is bounded by a minimum propagationdelay and a maximum propagation delay. The minimum propagation delay isthe amount of time required for a signal to travel from user terminal126 to satellite 118 (and a gateway) generally when satellite 118 isdirectly above user terminal 126. The maximum propagation delay is theamount of time required for a signal to travel from user terminal 126 tosatellite 118 when satellite 118 is located at a predetermined usefulhorizon of user terminal 126. The total delay is also affected by theposition of the gateway relative to the satellite, and may change thesatellite position at which maxima or minimums occur. In a similarmanner, some degree of timing uncertainty can arise for relative motionbetween a user terminal and base station 114 or other signal sources,although generally of lesser magnitude, depending on the relativemotion.

[0066] Resolving the timing uncertainty is necessary in order toproperly acquire access probe 330. Specifically, the PN code phase andtiming, that is, the time of the start of the PN code sequences, must beknown in order to despread the long and short PN codes used in formingaccess probe 330. This is done by correlating access probe 330 withvarious timing (and code as appropriate) hypotheses to determine whichtiming hypothesis is the best estimate for acquiring access probe 330.The timing hypotheses are offset in time (and frequency for Dopplereffects) from one another and represent various estimates of the timingof access probe 330, or of the PN codes used to generate the accesssignal. The hypothesis that generates the highest correlation withaccess probe 330, generally one that exceeds a predetermined correlationthreshold, is the hypothesis with the most likely estimate (assumed“correct” or appropriate) of the timing to use for that particularaccess probe 330. Once the timing uncertainty is resolved in thismanner, access probe 330 can be despread using the resolved timing andthe long and short PN codes according to well-known techniques.

[0067] V. System Timing for Access Probe Transmission

[0068] The usual access technique for an access signal is a slottedrandom access known as “slotted ALOHA.” According to this technique,communication system 100 establishes a regular timing structure on theaccess channel to coordinate access probe transmissions. FIG. 4 is atiming diagram depicting a typical timing structure for access signalsor probes in a conventional slotted random access channel 400. Channel400 comprises access slots 402, boundaries 404, guard bands 406 andaccess probes 408. Channel 400 is divided into time blocks of equalduration known as access slots 402 having boundaries 404. In a preferredembodiment, each access slot 402 includes a leading guard band 406A anda trailing guard band 406B to accommodate the timing uncertaintiesdescribed above.

[0069] When a user terminal desires to access communication system 100,that is, initiate or respond to communications, the user terminaltransmits access signal or probe 408 to gateway 122. Conventional accessprobe 408 includes an access preamble and an access message, and istransmitted by access channel transmitter 310 in user terminal 126 toaccess channel receiver 320 in gateway 122. In a conventional spreadspectrum system, the preamble and access message are both quadraturespread with a pair of short PN codes and channelized with the long PNcode. The preamble typically comprises null data, that is, all “1”s orall “0”s, or a pre-selected pattern of “1's” and “0's”. The preamble istransmitted first to provide access channel receivers with anopportunity to acquire access probe 408 prior to the access messagebeing sent. When access channel receiver 320 receives the preamble,access channel receiver 320 must despread it using the short PN codepair and the long PN code. Once the short PN and long codes aredetermined by access channel receiver 320, the access probe is referredto as being acquired. After the preamble has been transmitted for apredetermined period of time, the access message is transmitted byaccess channel transmitter 310. The access message is spread using thesame short PN code pair and long PN code used to spread the preamble.

[0070] The preamble must be of sufficient length so that access channelreceiver 320 has time to process the hypotheses and acquire the accessprobe before the access message is transmitted. Otherwise, accesschannel receiver 320 will still be attempting to acquire the accessprobe while the access message is being transmitted. In this case, theaccess message will not be properly received. The time required toacquire an access probe, referred to as acquisition time, variesdepending on how many receivers are used in parallel to process thehypotheses, how long the various code sequences are, the range of timinguncertainty in the signal transmissions, and so forth. In addition, thelength and frequency of repetition of the preamble is selected in orderto minimize collisions between access probes transmitted by differentuser terminals. Each of these factors are considered based on systemdesign considerations when determining the length of the preamble aswould be apparent.

[0071] Access probes of conventional design mutually interfere iftransmitted simultaneously. For this reason, only one conventionalaccess probe can be successfully received during one access slot on aslotted random access channel. Because access slots are not reserved forparticular users, a user can transmit during any access slot. The userthen waits for an acknowledgment from the receiver before transmittinganother message. If no acknowledgment is received after a predeterminedperiod, the user assumes that the access probe has collided with anaccess probe from another user, or simply not been received, andretransmits the access message.

[0072] Access slot duration (less guard bands) in a conventional slottedrandom access channel is selected to exceed the length of the longestpossible access probe. Conventional access probes are then transmittedso as to fall completely within one access slot 402. This arrangementreduces the likelihood of collisions to some extent. However, thisarrangement also causes a significant amount of access channel 400 to gounused. Because it is costly to add communication channels, it isdesirable to minimize the unused portion of any communication channel,especially one used to gain access to a system or setup communicationlinks.

[0073]FIG. 5 is a timing diagram for access probes in a slotted randomaccess channel according to a preferred embodiment of the presentinvention. In FIG. 5, conventional access probes 408 have been replacedby multi-part access probes 502 according to the present invention. Sucha multi-part access probe is disclosed in detail in a copending,commonly-owned application, filed Jun. 16, 1998, entitled “Rapid SignalAcquisition and Synchronization For Access Transmissions,” which issuedas U.S. Pat. No. 6,044,074, which is incorporated herein by reference.As described below, such multi-part access probes can partially overlapunder certain conditions. This technique not only significantly reducesthe unused portion of access channel 400, but also permits multipleaccess probes 502 to share the access channel 400 at substantially thesame time, at least for a certain period. One fundamental differencebetween the invention and conventional protocol 400 is that the preambleis initially spread with only short PN code pair, and later with boththe short PN code and long PN code. This allows access channel receiver320 to resolve the timing uncertainty using only short PN code pair 620.In contrast, conventional protocol 400 requires the use of both short PNcode pair 620 and long PN code 622 to resolve timing uncertainty.

[0074] VI. Protocol for Transmitting an Access Probe According to thePresent Invention

[0075]FIG. 6 illustrates a protocol or process structure 600 forgenerating an access probe 502 according to one embodiment of thepresent invention. In protocol 600, access probe 502 includes an accessprobe preamble (preamble) 604 and an access probe message (accessmessage) 606. According to the present invention, preamble 604 istransmitted in two stages: a first stage 508 and a second stage 510.Access message 606 is transmitted in a single message stage 512. Stages508, 510 and 512 are grouped into two parts for modulation purposes:first part 504 and second part 506. First part 504 includes first stage508, and is spread with a short PN code 620. Second part 506 includessecond stage 510 and message stage 512, and is spread with short PN code620 and a long PN code 622. In a preferred embodiment, short PN code 620is a pair of quadrature PN codes and is used to spread the signal usingwell-known techniques. In one embodiment, the PN code sequence used tospread a Q channel can be a delayed version of the PN code sequence usedto spread the I channel, although separate codes are preferred.

[0076] In first stage 508, preamble 604 of access probe 502 is spread byshort PN code 620 for a length of time sufficient to allow accesschannel receiver 320 to determine the timing of short PN code 620.Preamble 604 can comprise any bit pattern that facilitates acquisitionof access probe 502. In a preferred embodiment, the bit pattern forpreamble 604 is null data, such as a bit pattern of all ones, all zeros,or a pre-selected pattern of “1's” and “0's”. In order to facilitaterapid acquisition of access probe 502 by gateway 122, long PN code 632is not used to spread first stage 508.

[0077] In second stage 510, preamble 604 of access probe 502 is spreadby short PN code 620, as for first stage 508. Preamble 604 is alsospread by long code 622 to facilitate synchronization of the long codeby gateway 122. When user terminal 126 attempts an access on a specificaccess channel, long code 622 includes a mask associated with thataccess channel, creating a pseudo-orthogonal PN code. The gateway usesthe same mask to demodulate signals for that specific access channel. Bythe end of second stage 510, access channel receiver 320 should haveacquired access probe 502.

[0078] Access messages can be encoded in a similar fashion to data onthe typical traffic channels which is M-ary modulated using a set oforthogonal codes such as Walsh functions. The data could also bemodulated using single Walsh functions, although the timing uncertaintygenerally works against this approach.

[0079] In an alternative embodiment, during message stage 512 themessage data is modulated by one or more orthogonal codes selected froma set of orthogonal codes, then spread by short code 620, and spread bylong code 622. An exemplary set of orthogonal PN codes is disclosed in acommonly-owned copending U.S. patent application Ser. No. 08/627,831,entitled “Using Orthogonal Waveforms to Enable Multiple Transmitters toShare a Single CDM Channel”, which is incorporated herein by reference.

[0080] Two access probes 502 generated using protocol 600 can collide ormutually interfere under certain conditions. For example, two signalsmodulated with the same short PN code 620 will mutually interfere if thedifference in their arrival times at access channel receiver 320 is lessthan one-half of a chip, modulo 256 chips. Therefore, two access probes502 can collide if their first stages 508 are transmitted to be receivedwithin the same access slot 402.

[0081] Further, two signals modulated with the same short PN code 620and the same long code 622 will mutually interfere under certainconditions. Specifically, two signals modulated with the same short PNcode 620 and the same long PN code 622 will mutually interfere if thedifference in their arrival times at access channel receiver 320 is lessthan one-half of a chip, modulo 256 chips. Therefore, two access probes502 can mutually interfere if their second stages 510 are transmitted tobe received within the same access slot 402.

[0082] However, signals modulated with short PN codes 620 only do notcollide with signals also modulated with long PN code 622. Therefore,the first stage 508 of one access probe can occupy the same access slot402 as the second stage 510 and/or the message stage 512 of anotheraccess probe.

[0083] Further, signals modulated with one orthogonal code (when used)do not mutually interfere with signals modulated with other orthogonalcodes selected from the same set of orthogonal spreading codes.Therefore, the message stage 512 of one access probe can occupy the sameaccess slot 402 as the message stage 512 of another access probe.

[0084] Therefore, according to the present invention, access probes 502can share an access slot 402, or a portion thereof. Thus, when theslotted random access technique is observed for the first stage 508 ofeach access probe 502, and the arrival times of the second stages ofaccess probes 502 do not coincide as described above, communicationssignals modulated according to the protocol of FIG. 6 can partiallyoverlap, as shown in FIG. 5. This allows use of slot time that isotherwise wasted or unavailable. Thus, the present invention results inmore efficient usage of communications channels.

[0085] Furthermore, the length of each access slot has normally beendefined as the sum of the lengths of each portion of an access signal,that is the preamble and message portions, plus guard bands (if used)(stage 508+stage 508+512). This provides the number of slots over agiven time period that are available. The number of available accesschannels on a given frequency is limited by the number of short PNcodes. Together these facts provide the number of time slots in whichusers can attempt to access communication system 100. However, with thepresent invention, the number of access channels can be effectivelyincreased.

[0086] For example, the fact that portions or stages of access probescan overlap can be used to create multiple access channels. That is,access channels can be formed which are based on or use short PN codeswhose timing structure is shifted by a preselected time period dedicatedor used for the first portion of the preamble (short PN spread only).The channels use the same short PN codes time shifted from each othersuch that the various portions of adjacent access signals or probes thatcan be received do not coincide. An access probe can be received in onechannel while another channel receives another access probe that usesthe same short PN code but has a time offset, the length of the firstpreamble stage or greater, so that the two signals do not collide. Thereception of the second preamble stage and the message portion will notcause a collision in this scheme, and those portions do not need to beaccounted for directly in establishing the channel offsets. Thereceivers can establish the channels according to time shifted PN codesthey use for hypothesis in the signal acquisition and demodulationprocesses. Depending on the length of the time used for the time offsetsto assure preamble reception, and any desired guard bands, as before, itis estimated that at least two or there times as many channels can becreated in the same frequency space.

[0087] However, a preferred embodiment of the invention recognizes thatalternatively the total (fixed) length of each of the slots can bereduced to the period of the short PN code, plus guard bands or extratime as desired for system performance. Since the access probes shouldnot collide except for over this short period of time when the sameshort PN codes are used, longer time slots are not necessary todistinguish, acquire, and demodulate access signals. This allows agreater number of access slots per channel (also referred to as channelsin some systems) to in effect be created on the access channels orfrequencies. This technique provides for increased access channelcapacity and ease of access without increasing the complexity ofhardware or control systems used to create and monitor the accesschannels.

[0088] VII. Access Channel Transmitter

[0089]FIG. 7 is a circuit block diagram for an exemplary access channeltransmitter 310 for transmitting an access probe 502 according to theprotocol or signal structure of FIG. 6. Access channel transmitter 310includes a data modulator 702, PN code modulators 704, transmitter 706and antenna 708.

[0090]FIG. 8 is a flowchart describing the operation of the circuit ofFIG. 7. In a step 802, data modulator 702 modulates a carrier signal(baseband) of conventional design (not shown) with an access message toproduce a message stage 512 of second part 506 of access probe 502. In astep 804, PN code modulator 704A modulates a portion of the signalproduced by data modulator 702 using long PN code 622 to produce secondpart 504 of access probe 502. In a step 806, PN code modulator 704Bmodulates first part 504 and second part 506 of the signal produced byPN code modulator 704A using short PN code 620. In a step 808,transmitter 706 transmits access probe 502 via antenna 708 so that firstpart 504 of access probe 502 falls completely within one access slot402.

[0091] VIII. Access Channel Receiver

[0092]FIG. 9 is a circuit block diagram for an exemplary access channelreceiver 320 for receiving an access probe 502 according to the protocolof FIG. 6. Access channel receiver 320 includes a searcher 902,demodulators 904A-904N, and antenna 908. The two-stage architecture ofaccess channel receiver 320 is ideal for processing the multi-partaccess probe of the present invention in a pipeline manner, as describedbelow.

[0093] In operation, searcher 902 receives access probe 502 usingantenna 908 and acquires preamble 604. Preamble 604 is acquired byacquiring short PN code 620 and long PN code, as described above, anddespreading access probe 502. When searcher 902 has acquired preamble604, searcher 902 transfers the despread access probe to one of thedemodulators 904 (904A-904N). Demodulator 904 demodulates the despreadaccess probe to obtain access message 606.

[0094] Because preamble 604 and access message 606 are obtained byseparate functional units, they can occur simultaneously for differentaccess probes. That is, more specifically, a demodulator 902 candemodulate an access message of one access probe while searcher 902acquires the preamble of another access probe. This arrangement isideally suited for more efficient use of overlapping multi-part accessprobes according to the present invention. As discussed above, becausean access signal that is not successfully received can be sent againbefore an entire conventional access period has passed, even unacquiredor failed access signals can more efficiently gain access to thecommunication system. In addition, when there are additional offsetaccess channels provided or shorter time slots being used, thelikelihood of non-acquisition decreases along with the time to re-sendand acquire access signals.

[0095] IX. Conclusion

[0096] The previous description of the preferred embodiments is providedto enable any person skilled in the art to make or use the presentinvention. While the invention has been particularly shown and describedwith reference to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the spirit and scope of theinvention. For example, the invention is equally suited fortransmissions other than access channel transmissions that are spreadwith multiple code sequences.

What we claims as our invention is:
 1. A system for transmittingmulti-part access probes over a slotted random access communicationschannel having a plurality of access channel slots, each of said accessprobes including an access message, the system comprising: a firstmodulator for modulating a first part and a second part of the accessprobe with a short pseudonoise sequence; a second modulator formodulating said second part of the access probe with a long pseudonoisesequence; a data modulator for modulating said second part with theaccess message; and a transmitter for transmitting the access probe sothat said first part falls within one of the access channel slots. 2.The system of claim 1 , wherein the length of said short PN sequence is2⁸ chips.
 3. The system of claim 1 , wherein said short PN sequencecomprises a pair of quadrature short pseudonoise sequences.
 4. A methodfor transmitting a plurality of access signals over at least one accesschannel, each including preamble and message portions with the preamblehaving first and second stages, comprising the steps of: modulating thefirst stage and second stage of the preamble by a first signal;modulating the second stage of the preamble also by a second signal;modulating the message with said first signal and said second signal;and transmitting said access signal in the form of said modulated firststage, said modulated second stage, and said modulated message so thatsaid preamble falls within one of a plurality of preselected time slotswhose length corresponds substantially to that of said first stage. 5.The method of claim 4 , wherein more than one access signal istransmitted in time such that a second stage or message portion overlapsthe first stage of one or more other transmitted access signals.
 6. Themethod of claim 4 , further comprising guard bands forming boundariesfor said preselected time slots.
 7. The method of claim 4 , wherein saidmodulated first stage of the preamble is transmitted for a sufficienttime for a receiver to acquire a timing of said first signal.
 8. Themethod of claim 7 , wherein said modulated second stage of the preambleis transmitted for a sufficient time for a receiver to acquire a timingof said second signal.
 9. The method of claim 4 , wherein said firstsignal is a pair of quadrature spreading, pseudonoise sequences.
 10. Themethod of claim 9 , wherein the length of said pseudonoise sequences is2⁸ chips.
 11. The method of claim 9 , wherein said second signal is achannelizing pseudonoise sequence.
 12. The method of claim 11 , whereinsaid access signal comprises a message following said preamble, saidmessage modulated by said first code sequence and said second codesequence.
 13. A method for using an access signal in a wirelesscommunication system comprising: transmitting an access signal includinga preamble and a message, said preamble having a first stage of apredetermined first length and a second stage, said first stage havingdata modulated by a first signal, said second stage having datamodulated by a second signal and said first signal; and receiving saidaccess signal over an access channel divided into signal reception timeslots that are substantially the same length as said first stage. 14.The method of claim 13 , wherein the first stage of the preamble iscomprised of null data.
 15. The system of claim 13 , wherein the secondstage of the preamble is comprised of null data.
 16. The method of claim13 , wherein said first signal and said second signal are PN sequences.17. A method for using an access signal in a wireless communicationsystem comprising: transmitting an access signal including a preambleand a message, said preamble having a first stage of a predeterminedfirst length and a second stage, said first stage having data modulatedby a first signal, said second stage having data modulated by a secondsignal and said first signal; and receiving said access signal over aplurality of an access channels divided into signal reception time slotsthat are time offset from each other by a period substantially the samelength as said first stage.
 18. The method of claim 17 , wherein thefirst stage of the preamble is comprised of null data.
 19. The method ofclaim 17 , wherein said first signal and said second signal are PNsequences.
 20. A method for acquiring a transmission at a receiver froma transmitter, the transmission having a preamble, the preamble having afirst stage and a second stage, the method comprising the steps of:performing a coarse search on the transmission received by the receiverduring the first stage of the preamble, wherein the first stage of thepreamble is modulated by a first signal, said coarse search to determinea timing offset of said first signal; performing a fine search on thetransmission received by the receiver during the second stage of thepreamble, wherein the second stage of the preamble is modulated by saidfirst signal and a second signal, said fine search to determine a timingoffset of said second signal, wherein said timing offset of said secondsignal is determined using said first signal and said timing offset ofsaid first signal; and demodulating the transmission using said firstsignal, said second signal, said timing offset of said first signal, andsaid timing offset of said second signal.